1. Field of the Invention
The present invention relates to surface coverings such as coatings for exterior building materials such as exterior cladding including siding material, roofing shingles, roofing membranes, and the like.
2. Brief Description of the Prior Art
It is well known that surfaces with dark colors will absorb significant amounts of solar heat from solar radiation, while light or white colors will reflect more solar radiation and hence produce lower surface temperatures under the sun. This is particularly true for a building envelope or building claddings, such as roofing and siding materials. In this case darker colors can absorb significant amount of solar heat and thus increase the energy required for cooling the interior during hot seasons.
Reduced energy consumption is an important national goal. For example, the State of California has a code requirement that all commercial roofing materials in low-slope applications need to exceed a minimum of 70% solar reflectance in order to meet California's energy budget code. Also, in order to qualify as Energy Star® roofing material, a roofing membrane needs to exceed 65% solar reflectance.
To reduce the solar heat build-up at exposed building surfaces or roofing assemblies, one may consider using light colors, or the so-called “cool coloring technology” with reflective properties in the near infrared of solar radiation to reduce solar heat absorption. However, such solutions will result in energy penalty during cold seasons, where heating energy demand can be increased due to loss of solar heat absorption. While in some climates, such as southern California, solar heat resistance is the prime if not exclusive consideration, in most of the United States there is a significant energy demand for heating during much of the year.
Thus, it is advantageous to have a colored surface that can not only provide aesthetic values to a structure, but also optimize the solar energy absorption/reflection in order to maximize whole building energy efficiency by adjusting its solar reflectance in response to the change of surrounding temperatures.
This problem has been addressed by using thermochromatic or thermochromic colorants that can change from dark to light colors as surface temperature changes.
For example, U.S. Pat. No. 5,571,596 discloses a roofing shingle which preferably includes a topcoat of a thermo-chromatic pigmented paint which changes color with temperature, lightening in strong sunlight to reflect heat and darkening during cool shady days to absorb heat. U.S. Pat. No. 6,500,555 discloses thermochromic laminates for use in roofing and siding to control the temperature of an underlying structure. U.S. Patent Application Publication No. 2005/0064175 discloses microencapsulated thermomorphic materials that can be applied to roofs in situ or to the exterior surfaces of shingles, as well as electorphoretic panels that can be applied over existing roofs to achieve a controllable color change. However, thermochromatic colorants can result in color change of the surfaces. Such color changes may not be desirable for exposed building surfaces where aesthetic appearance is important. U.S. Pat. No. 6,647,688 discloses providing a luminescent material in vinyl siding or trim so that the siding or trim may absorb light during the daytime and glow at night.
Depending on local temperature fluctuations, a surface covered with a substance containing a thermochromic pigment may experience color variation that the consumer may find very objectionable. Also, it is known that thermochromatic colorants are generally not sufficiently lightfast for use in applications subject to outdoor weathering exposure.
Other approaches to building temperature control include systems with retractable film or textiles mounted to the rooftop or claddings to manage the solar heat absorption in order to maximize building energy consumption during hot/cold seasons.
In other systems, water circulation or water evaporation have been used to control the surface temperature of exterior structures.
U.S. Pat. No. 6,446,402 discloses thermochromic window materials that substantially block solar radiation when the ambient temperature is high and allow sunlight to enter when the ambient temperature is low.
There is a continuing need for exterior surfacing material such as roofing materials that have improved solar reflectance while providing and maintaining an attractive appearance. Further, there is a continuing need for surfacing materials that contribute to energy saving in both the cooling season and the heating season.